Chance of dry thunderstorms in North Bay, as remnants of tropical storm drift in

FILE -- Lightning strikes above the Geysers area of northern Sonoma County, early Thursday, July 4, 2013 near Geyserville Calif.

FILE -- Lightning strikes above the Geysers area of northern Sonoma County, early Thursday, July 4, 2013 near Geyserville Calif.

Photo: Kent Porter, Press Democrat Via AP

Photo: Kent Porter, Press Democrat Via AP

Image
1of/3

Caption

Close

Image 1 of 3

FILE -- Lightning strikes above the Geysers area of northern Sonoma County, early Thursday, July 4, 2013 near Geyserville Calif.

FILE -- Lightning strikes above the Geysers area of northern Sonoma County, early Thursday, July 4, 2013 near Geyserville Calif.

Photo: Kent Porter, Press Democrat Via AP

Chance of dry thunderstorms in North Bay, as remnants of tropical storm drift in

1 / 3

Back to Gallery

Forecasters are warning of thunderstorms overnight in the North Bay as remnants of Tropical Storm Ivo drift north.

The National Weather Service reports the eye of the weakened and wrung-out storm that hit Baja California on Monday was a couple hundred miles off the coast of Big Sur at 1 p.m. Tuesday.

As the system moves north and reaches shore late tonight, it's expected to cause instability in the atmosphere over Sonoma and Napa Counties where there's a 15 to 20 percent chance of thunderstorms between midnight Tuesday and 9 a.m. Wednesday.

"The chance of light rain is even lower than thunderstorms," says NWS forecaster Spencer Tangen. "The reasons for that is the lower atmosphere is pretty dry. Any rain that falls will dry up before reaching the ground."

Tangen says there's an even smaller chance for thunder and lightning in Marin and San Francisco counties, but it's "not zero."

Moisture from the storm was collecting in the upper levels of the atmosphere over the greater San Francisco Bay Area Tuesday afternoon, as wispy high clouds gathered. Tangen says cloud cover will increase into the evening and through Wednesday morning. Humidity levels are also expected to increase with muggy, tropical-like conditions likely. Afternoon highs on Tuesday and Wednesday are expected to be in the high-70s on the coast and in the 90s inland

By Wednesday afternoon, the system will have exited the region with high clouds clearing, but the typical summer fog will likely linger at the coast.

The second half of the week is forecasted to be cooler with a strong onshore flow and coastal fog. On Thursday and Friday, coastal areas will be in the low-70s and inland areas in the mid-to high-90s. The weekend will be the start of a warmup with coastal spots forecast to heat up into the high-70s and inland valleys into the 90s.

Amy Graff is a digital reporter for SFGATE. Send her weather tips and photos at agraff@sfgate.com.